1. Field of the Invention
The invention relates to variable-format web-fed offset printing machines and a method of producing variable-format surfaces.
2. Description of the Related Art
U.S. Pat. No. 5,819,657 discloses the production of plastic sleeves with various wall thicknesses, which are fitted to core cylinders as intermediate sleeves and bear flexographic or gravure printing plates with different circumferential lengths. The intermediate sleeves are constructed from an inner layer which, under air pressure, expands in the radial direction and compresses a following compressible layer. The compressible layer is followed by a solid transition layer which bears an incompressible bridge layer of different thicknessxe2x80x94depending on the format. A printing sleeve with a printing plate may be fitted to a cylindrical terminating layer over the bridge layer.
The intermediate sleeves can be pushed onto a core cylinder by means of compressed air, and the printing sleeves can be pushed onto the intermediate sleevesxe2x80x94likewise over an air pad produced with compressed air. For mounting the intermediate sleeves, a compressed-air connection is provided at the end of the core cylinder, and holes in its circumferential surface. On the intermediate sleeves, air channels are provided in the bridge layer, parallel to the axis of rotation of the cylinder, and have pressure connections at the end annular surface and lead to the circumferential surface of the terminating layer via radial holes which are spaced apart axially from one another.
In order to shrink the intermediate sleeves and the printing sleeves on axially, and to remove them, it is disadvantageous to have to provide two separate air supplies on the printing machine. The air channels running axially in the bridge layer are complicated to produce and require a minimum wall thickness of the bridge layer.
U.S. Pat. No. 5,706,731 discloses flexography cylinders with hollow supporting cylinders, which are provided with a central air supply and, in the vicinity of the insertion end, have radial holes on the circumference as connecting channels to the centre, to which compressed air can be applied. Intermediate sleeves are pushed onto these supporting cylinders and, at their one end, likewise have radial holes, in order to be able to use the centrally supplied compressed air to mount printing plates onto the circumferential surface as well. In order that the operation of shrinking the intermediate sleeves on axially is not disrupted as soon as their air channels come into alignment with the air supplies of the supporting cylinders, the intermediate sleeves are provided with rotatable closure rings. By means of the latter, the air outlet at the circumferential surface can be closed and, as soon as the intermediate sleeve has been shrunk on completely, the path for the compressed air to the circumferential surface of the intermediate rings can be opened by rotating the closure rings, by which means a printing plate can be shrunk onto the intermediate sleeve.
The switchable closure rings have to be produced precisely and make the intermediate sleeves more expensive.
The object of the invention is to make economic, variable-format printing with web-fed offset printing machine possible by means of simply constructed and simply mounted sleeves on the printing-unit cylinders.
According to a first aspect of the invention, each printing unit cylinder includes
(a) a core cylinder having an outer circumferential surface and means for supplying compressed air to the surface;
(b) a carrier layer having an inner surface which rests on the surface of the core cylinder;
(c) a compressible intermediate layer over the carrier layer;
(d) a transition layer over the compressible intermediate layer;
(e) a variable thickness bridging layer over the transition layer; and
(f) a covering layer on the variable thickness bridging layer, which covering layer is suitable for receiving a functional surface, such as a printing plate or a rubber blanket, which can be pushed on axially and shrunk on radially.
According to a second aspect of the invention, each printing unit cylinder includes the elements (a) to (e) above, however the covering layer is deleted and the bridging layer is suitable for receiving a functional layer which can be inseparably fitted to the bridging layer.
According to a third aspect of the invention, a variable format cylinder for a web-fed offset printing machine is produced by the following steps
(a) providing an intermediate sleeve having an open interior with axial end openings, an outer circumferential surface, and holes extending between the open interior and the outer circumferential surface;
(b) closing the open ends in an air-tight manner;
(c) supplying compressed air to the interior so that air emerges from the holes on the outer circumferential surface of the intermediate sleeve to form an air pad;
(d) pushing a functional sleeve axially onto the outer circumferential surface of the intermediate sleeve while the said compressed air is being supplied;
(e) switching off the compressed air so that the functional sleeve shrinks radially onto the intermediate sleeve;
(f) providing a core cylinder having an outer circumferential surface and means for supplying compressed air to the outer circumferential surface of the core cylinder;
(g) supplying compressed air to the outer circumferential surface of the core cylinder;
(h) pushing the intermediate sleeve with the functional sleeve onto the outer circumferential surface of the core cylinder while the compressed air is being supplied; and
(i) switching off the compressed air so that said intermediate sleeve shrinks radially onto the core cylinder.
The invention makes a format change possible which can be carried out quickly and simply by the printer himself.
By means of the invention, the investment costs involved in procurement are advantageously reduced, since the plastic sleeves for different formats can be bought in at any time.
It is also particularly advantageous that the users of conventional and digital web-fed offset printing can print with variable cut lengths in accordance with their specific requirements, and are therefore able to run economic production, matched to the job, with the respective optimum paper waste.
As a result of the advantageous configuration according to the invention, the intermediate sleeves have a low weight, as a result of which they can be replaced easily and ergonomically.
As a result of the beneficial choice of materials with a low thermal conductivity and heat capacity, the sleeves according to the invention are advantageously also suitable for computer-to-press technologies, in which the printing plates have images set on them within the printing machine and lead to heating of the surfaces, such as in the case of the thermal transfer processes.
The particularly advantageous shaping of the intermediate sleeve, which forms a detachable shrunk seat on a core cylinder and has a further detachable joint for the functional sleeve, means that the printing plate or rubber-blanket sleeves can be replaced cost-effectively as required.
By means of an advantageous production method for the intermediate sleeves, which provides for the functional surface to be inserted into a cylindrical mould and for the bridging layer to be foam-filled directly, more accurate production tolerances can be achieved, the cost can be reduced, and the omission of the compressed-air holes in this method leads to a further simplification of production.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.